The present invention relates to a novel binder composition for agglomerating particulate materials, a novel process for agglomerating particulate materials using said binder composition, and to the agglomerated products produced from said process. The process is particularly useful for agglomerating metallic ores such as iron ore.
Agglomeration is commercially used in industries where materials are encountered in a form which is too finely divided for convenient processing or handling. Thus, there is a need to upgrade the size, density and/or uniformity of finely divided particles for more efficient handling, processing or recovery. Agglomeration is particularly useful in the metal refining industry, where the concentrate ore encountered is typically finely divided.
Many processes for the agglomeration of particles, especially metallic particles, are known in the art. In the mining industry it is common practice to agglomerate or pelletize finely ground mineral ore concentrate to facilitate shipping of the ore. After the mineral ore has been mined, it is frequently wet ground, though not always the case, and screened to remove large particles which can be recycled for further grinding. The screened mineral ore is known in the art as "concentrate".
After screening, a binding agent is added to the wetted mineral ore concentrate and the binder/mineral ore composite is conveyed to a balling drum or other means for pelletizing the ore. The binding agent serves to hold or bind the mineral ore together until after firing. After the balling drum operation, the pellets are formed, but they are still wet. These wet pellets are commonly referred to as "green pellets" or "green balls". These green pellets are thereafter transported to a kiln and heated in stages to a end temperature of about 2400.degree. F.
For many years, bentonite clay was the binding agent of choice in the pelletizing operations for mineral ore concentrates. Use of bentonite as a binding agent produces balls or pellets having a very good wet and dry strengths and also provides a desired degree of moisture control. Use of bentonite does, however, have several disadvantages. Initially, bentonite adds to the silica content of the pellets when the ore pellets are fired at a temperature of 2400.degree. F. or higher. Higher amounts of silica are not desirable because silica decreases the efficiency of blast furnace operations used in smelting the ore.
The use of bentonite to form pellets of mineral ore concentrates can also add alkalis which are oxides of, for example, sodium and potassium. The presence of alkalis in the blast furnace causes both the pellets and coke to deteriorate and to form scabs on the furnace wall, which increases fuel consumption and decreases the productivity of the smelting operation.
Organic binders have proven to be an attractive alternative to bentonite because organic binders do not increase the silica content of the ore and they impart physical and mechanical properties to the pellets comparable with those of bentonite. Organic binders also burn out during ball firing operations thus causing an increase in the microporosity of the pellets. Accordingly, the pore volume and surface/mass ratio of the formed pellets produced using organic binders is larger than that of pellets produced using bentonite. Due to the larger surface area and increased permeability of the pellets produced using organic binders, the reduction of metallic oxides such as iron oxide is more efficient than with pellets prepared with bentonite.
Examples of some commonly mentioned organic binders include polyacrylate, polyacrylamide and copolymers thereof, methacrylamide, polymethacrylamide, cellulose derivatives such as alkali metal salts of carboxymethyl cellulose and carboxymethylhydroxyethyl cellulose, poly (ethylene oxide), guar gum, dairy wastes, starches, dextrins, wood related products, alginates, pectins, and the like.
U.S. Pat. No. 4,751,259 discloses compositions for iron ore agglomeration which comprise 10-45% by weight of a water-in-oil emulsion of a water soluble vinyl addition polymer, 55-90% by weight of a polysaccharide, 0.001-10% by weight of a water soluble surfactant and 0-15 weight % of Borax.
U.S. Pat. No. 4,948,430 discloses a binder for the agglomeration of ore in the presence of water, which comprises 10% -90% of a water soluble sodium carboxymethylhydroxyethyl cellulose and 10% to 90% of sodium carbonate.
U.S. Pat. No. 4,288,245 discloses pelletization of metallic ores, especially iron ore, with carboxymethyl cellulose and the salt of a weak acid.
U.S. Pat. No. 4,863,512 relates to a binder for metallic containing ores which comprises an alkali metal salt of carboxymethyl cellulose and sodium tripolyphosphate.
European Patent Application Publication No. 0 376 713 discloses a process for making pellets of particulate metal ore, particularly iron ore. The process comprises mixing a water-soluble polymer with the particular metal ore and water and pelletizing the mixture. The water-soluble polymer may be of any typical type, e.g., natural, modified natural or synthetic. The mixture may optionally comprise a pelletizing aid which may be sodium citrate.
Organic binder compositions, such as those mentioned above, are not, however, without their own disadvantages. While they are effective binders, they generally do not impart adequate dry strength to the pellets at economical use levels. Thus, there is an ongoing need for economical binders with improved properties.